1. Field of the Invention
This invention relates to an ultrasonic diagnostic system and an ultrasonic diagnostic method, and more particularly to an ultrasonic diagnostic system and an ultrasonic diagnostic method which enable a lesion on a living tissue, such as the heart, to be diagnosed by a noninvasive measurement using ultrasonic, by which local degeneration of myocardial structure in any region of the heart wall is identified, visually displayed and thereby enabled to be diagnosed.
2. Description of the Related Art
In diagnosing heart diseases due to the malfunctioning of cardiac muscle, such as hypertrophic cardiomyopathy, dilated cardiomyopathy or ischemic cardiac diseases including myocardial infarction, the tissue characteristics of cardiac muscle should be identified. While known methods include diagnosis using ultrasonic integrated backscatter besides invasive biopsy, available diagnostic apparatuses cannot follow variations in the region of interest, because they fix the region of interest in the heart wall. Accordingly these apparatuses can mainly sense signals of around 1 Hz due to density variations of myocardial fibers, but can hardly diagnose structural changes in cardiac muscle. The present invention is intended to provide an effective solution to this problem.
Diagnosis of Heart Diseases:
Ultrasonic diagnostic methods for heart diseases extensively used today are mostly based on morphological aspects of the heart, such as evaluation of its wall thickness or the cardiac output, but can hardly be helpful in the tissue characterization of cardiac muscle. Although it is essential to know the tissue characteristics of cardiac muscle in diagnosing heart diseases including hypertrophic cardiomyopathy, dilated cardiomyopathy and ischemic cardiac diseases such as myocardial infarction, tissue characterization of cardiac muscle requires biopsy of cardiac muscle. Since this is an invasive technique, it imposes heavy physical and mental burdens on the subject, and therefore cannot be repetitively applied. On account of these background circumstances, there is a keen call for a noninvasive tissue characterization method applicable to cardiac muscle.
It is already known that observation of a heart suspected of obsolete myocardial infarction or dilated cardiomyopathy by a B-mode image or an M-mode image gives high brightness of echoes, namely high intensity of ultrasonic reflected, from the heart wall. However, it is difficult to quantitatively assess it on the image.
On the other hand, ultrasonic integrated backscatter (IB) from the heart wall is attracting note as a promising evaluation method for quantitative tissue characterization of the heart. Ultrasonic IB is measured as an average reflective power of ultrasonic from a given region in the tissue. The intensity of IB from the heart wall is known to manifest cyclic variations (CV) matching the pulsation of the heart, falling in the systole and rising in the diastole. Studies have been made on such cyclic variations of IB from many different aspects.
Physiology of Cardiac Muscle:
The mesh structure (or network structure) of fascicles of cardiac muscle fibers (each of about 50 fibers) finely varies between the systole and the diastole. The honeycomb-liked network structure, comprising lozenge units, is pulled in the diastole to crush each lozenge in shape. This changes the inclination of the face reflecting the ultrasonic. As the lozenges are crushed in the diastole, the ultrasonic coming vertically on the cardiac muscle fibers are more readily scattered, resulting in an increased backscattering intensity. In the systole on the other hand, each lozenge recovers its original full shape, and the inclination of the face reflecting the ultrasonic coming vertically on the cardiac muscle fibers is increased to make it more difficult for the ultrasonic to be scattered, resulting in a decrease of backscattering intensity IB.
There are clinical reports that the amplitude of the CV of IB is smaller in a patient of a cardiac disease such as myocardial infarction, hypertrophic cardiomyopathy or dilated cardiomyopathy than in a healthy person and the baseline of the ultrasonic backscatter IB is higher. Regarding this CV of IB, Hete et al., measuring the IB from extracted chicken skeletal muscle, demonstrated that the IB intensity rose when the muscle was passively extended, and attributed variations in IB level due to the extension of muscle to changes in the orientation of intercellular substance. Wickline et al., measuring the IB from the heart wall of a dog subjected to thoracotomy, studied the physical characteristics of the cardiac muscle by using a three-element Maxwell model. Their conclusion was that that the cardiac CV of IB could be attributed to variations in acoustic impedance accompanying the extension and contraction of the cardiac muscle.
Principle of Measurement of IB from Heart Wall:
The ultrasonic backscatter IB can be calculated by [Equation 1] below as the average power of ultrasonic reflected by a given region in the depthwise direction in the object.
                                          IB            0                    ⁡                      (            t            )                          =                  10          ⁢                                          ⁢                      log            10                    ⁢                      1                          Δ              ⁢                                                          ⁢                              D                ⁡                                  (                  t                  )                                                              ⁢                                    ∫                                                D                  0                                ⁡                                  (                  t                  )                                                                                                  D                    0                                    ⁡                                      (                    t                    )                                                  +                                  Δ                  ⁢                                                                          ⁢                                      D                    ⁡                                          (                      t                      )                                                                                            ⁢                                                                                                  z                    ⁡                                          (                                              t                        ,                        D                                            )                                                                                        2                            ⁢                                                          ⁢                              ⅆ                D                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
Here, z(t, D) is the orthogonal detection signal of the reflected signal, Do(t) the distance of ultrasonic propagation to the region of interest (ROI) at time t, and ΔD(t) the width of ROI.
FIG. 18 schematically illustrates an IB measuring system. A region of interest (ROI) 22 is set in a heart wall 21. Ultrasonic is transmitted from an ultrasonic probe 23 to the heart wall 21 at a repetition cycle of ΔT. The resultant reflected signal is detected with an orthogonal detector 24. The orthogonal detection signal z (t, D) thereby obtained is subjected to amplitude squaring. And, the time signal IBo(t) of the IB value is obtained by integrating with an integrator 25 the signal portions at different points of time from the region of interest (ROI) 22 according to [Equation 1] above.
In the measurement of IB from the heart wall in the scenes of medical practice today, dozens of frames of B-mode tomograms are taken, and the examiner sets the position of ROI in each frame to obtain the IB. However, as the heart wall moves translationally along with pulsation, and the wall thickness varies with the myocardial extension and contraction, the position and size of the ROI should be varied from one point of time to another, and therefore it is difficult by this method to measure the IB from the same region of the heart wall all the time.
Ultrasonic Integrated Backscatter from Cardiac Muscle:
Ultrasonic integrated backscatter IB from cardiac muscle is known to cyclically vary along with the pulsation of the heart (varying at a low frequency of about 1 Hz as the heart pulsates about once per second). In recent years, the function to measure such cyclic variations has come to be incorporated into ultrasonic diagnosing apparatuses for general medical purposes as well, and used for the tissue characterization of cardiac muscle. As cardiac muscle relaxes and extends in the diastole and contracts in the systole, (1) the number of cardiac muscle fibers (the number of scatterers) per unit volume varies (the density of cardiac muscle fibers varies) and at the same time (2) the intensity of the integrated backscatter also varies along with structural changes of cardiac muscle (for conventional methods of integrated backscatter, see References 1 through 4).
However, as the size and position of the region of interest set in the heart wall is assumed to be invariable during one beat for the expedience of the processing of calculation in IB measurements conducted with conventional ultrasonic diagnosing apparatuses (see for References 1 and 2), in effect the sum of the density variation under (1) and the intensity variation of the integrated backscatter within the region of interest under (2) ((1) +(2)) is measured. Of these factors, the density variation under (1) is 30% less in the systole than in the diastole, and this corresponds to 10 dB in scattering wave power. In such a conventional IB measurement, the density variation can be considered the dominant factor. Therefore, by the convention technique, only the variation in the density of scatterers within the region of interest is measured.
Phased Tracking Method:
Viewed from the aspect of high precision measurement of a blood vessel disease for instance, the conventional echocardiography M-mode has a resolution of only 1 mm or so at most. Similarly, when the vibration of an aorta is determined as a displacement velocity by the conventional Doppler method, the conditions for accuracy are satisfied theoretically, but the pulsation of the blood vessel has so significant an influence that it is difficult to extract a minute vibration superimposed over this relatively large amplitude. In view of this difficulty, researchers including the present inventors developed a phased tracking method whereby such a minute vibration on the beating heart or large blood vessel would be remotely measured ultrasonically to enable the elasticity modulus of the blood vessel wall on any selected spot could be calculated. Thus it was made possible to accurately diagnose the susceptibility of the content of an atheroma to rupture (see References 5 through 10). This phased tracking method will be outlined below.
The phased tracking method is a new bioinstrumentation for measuring minute vibration velocities of the heart wall and the blood vessel wall. It makes possible accurate measurement of vibrations of 500 Hz or less and 0.01 mm and changes of the wall in the order of 10 microns. By this method, for instance, minute velocities at a plurality of measurement points positioned between the intramural layers or on the wall of an artery by the ultrasonic Doppler method, and the minute velocities at the measurement points are subjected to time integral thereby to calculate positional changes over time at the measurement points. Since positional changes over time at the measurement points reveal variations in layer thickness, from which the elasticity modulus of the layer can be measured, it is made possible to estimate the susceptibility to rupture (Reference 6).
In practice, as shown in FIG. 19, measurement point (i) is set in the wall of an artery on an ultrasonic beam 26, and measurement point (i+1) is set in the next depth. Then, the minute vibration velocities vi(t) and vi+1(t) are measured for the measurement points, and the differences between the two velocities are subjected to time integral, thereby thickness change Δh(t) is determined which is thickness change of the layer between the measuring points (i) and (i+1) in the arterial wall. Incidentally, reference numeral 27 denotes plaques.
The References are listed below.    1. U.S. Pat. No. 4,867,167    2. U.S. Pat. No. 4,803,994    3. U.S. Pat. No. 4,688,428    4. U.S. Pat. No. 4,470,303    5. Japanese Patent Gazette (Patent Laid-Open No. 10-5226)    6. Japanese Patent Gazette (Patent Laid-Open No. 2000-229078)    7. U.S. Pat. No. 5,840,028    8. Kanai, H., Hasegawa, H., Chubachi, N., Koiwa, Y. and Tanaka, M., “Noninvasive evaluation of local myocardial thickness in heart wall and its color coding”, IEEE transaction UFFC, 1997; 44:752-768    9. Hasegawa., H, Kanai., H, Hoshimiya., N, Chubachi, N., Koiwa, Y., “Accuracy evaluation in the measurement of a small change in the thickness of arterial walls and the measurement of elasticity of the human calotid artery”, Jpn. J. Appl. Phys. 1998; 37:3101-3105    10. Kanai, H., Koiwa, Y., Zhang J., “Real-time measurements of local myocardium motion and arterial wall thickening”, IEEE transaction UFFC, 1999; 46:1229-1241
Earlier studies on ultrasonic backscatter IB took note only of the difference between the maximum and the minimum of IB in one cardiac cycle, but paid no sufficient attention to the variation of the IB value at each point of time in one cardiac cycle.